Bisphosphonates (BPs, FIG. 1) are non-hydrolyzable pyrophosphate analogs with high affinity to hydroxyapatite due to their ability to create bidentate or tridentate chelates with calcium ions. Consequently, Bisphosphonates leads to strong interactions with dentin, enamel and bones. Up to date, the wide clinical use of approved Bisphosphonates is for treatment of bone diseases associated with bone fragmentation, such as bone malignancies, osteoporosis, Paget's disease, etc. In addition to their strong affinity to calcium in the bone mineral, Bisphosphonates (especially those bearing OH) accelerate osteoblasts action, while strongly inhibiting osteoclasts, thus contributing to enhanced bone formation.
It was recently found that certain Bisphosphonates induced reduction in bone resorption, inhibited tumor growth and metastases formation, but had little effect on metastases already present in the bone. Hence, it is important to give prophylactic Bisphosphonates whenever skeletal metastases formation is a possible outcome of the cancer.
Numerous attempts to deliver drugs to the bones, via direct conjugation to Bisphosphonates, have been accomplished. A recent work reported a typical conjugation of Bisphosphonates to drugs via an anhydride with a phosphate bridge (Karpeisky, A., Zinnen, S. Bone targeted therapeutics and methods of making and using the same. US Pat Appl. 2009/0227544). However, the utilization of Bisphosphonates as bone-seeking agents has been much more successful in the field of bone-imaging. Complexes of Bisphosphonates with Tc and Re are widely used for the imaging of bone metastases using single photon emission computed tomography (SPECT) or planar scintigraphy. Recent studies attempted to use direct conjugates of Bisphosphonates with far-red and near IR fluorescent dyes, for non-isotopic imaging of bone turnover (McKenna, C. E., Sun, S., Blazewska, K. M., Kashemirov, B. A., Roelofs, A. J., Coxon, F. P., Rogers, M. J., Ebetino, F. H. Synthesis of novel risedronate and related conjugates with Rhodamine Red-X and Alexa Fluor 647: New fluorescent probes for bone active drugs. Abstracts of Papers, 238th ACS National Meeting, Washington, D.C., United States, August 2009).
Due to their highly hydrophilic properties, Bisphosphonates exhibit low skeletal bioavailability (around 1%). Over the years, most of the studies in this field were focused at increasing the Bisphosphonates uptake. Bisphosphonates have been conjugated to proteins, peptides and biocompatible polymers or encapsulated in various polymeric formulations, to allow better absorbing and slow-release of these drugs (Karavas, E., Koutris, E., Politis, S., Samara, V., Bikiaris, D. Pharmaceutical compositions containing bisphosphonates. WO 2009/018834). Some non-hydroxy Bisphosphonate conjugations to nanoparticles have also been investigated, but in most cases where Bisphosphonate-polymer conjugates were prepared, the Bisphosphonate was coupled to an existing polymer, usually via a spacer arm.
Bones and teeth are microstructurally and compositionally complex containing both organic and inorganic constituents. Common to these hard tissues are hydroxyapatite and collagen. The biocompatibility of synthetic hydroxyapatite is well documented, making it an attractive candidate as a biomaterial. Since hard tissues are composites they exhibit physical properties, which cannot be realized solely by a mineral constituent.
Thus, generation of such hard tissues with desired physical properties requires combination of the mineral with polymeric component. Greish and Brown described the formation of biocompatible organic-inorganic composites by reaction between tetracalcium phosphate and poly(vinyl phosphonic acid) (Greish, Y. E.; Brown, P. W. Chemically formed HAp-Ca poly(vinyl phosphonate) composites. Biomaterials 2001, 22, 807-816).
Schöller et al described the formation of hybrid particles via mineralization of calcium phosphate on the surface of copolymer particles composed of poly(vinylphosphonic acid) or poly(vinylbenzylphosphonic acid) and polystyrene (Schöller, K.; Ethirajan, A.; Zeller, A. Landfester, K. Biomimetic rout to calcium phosphate coated polymeric nanoparticles: Influence of different functional groups and pH. Macromolecular Chemistry and Physics 2011, 212, 1165-1175).
Mou et al described, for the first time, the synthesis and use of phosphorous-containing monomer for dental applications (Mou, L.; Singh, G.; Nicholson, J. W. Synthesis of a hydrophilic phosphonic acid monomer for dental materials. Chemical communication 2000, 345-346). They reported that the incorporation of a phosphonic function into monomer structures results in an increase biocompatibility and adhesion to the tooth, due to chelation with calcium ions in the tooth surface. Later on, diverse acrylic monomers containing phosphoric or phosphonic acids were prepared and evaluated as self-etching adhesive system for bonding of resin composite to enamel or dentin.
Alendronate, a commercial Bisphosphonate compound containing terminal primary amino group, was bound to various compounds and particles by the interaction of the primary amine group with appropriate functional groups. For example, Rayment et al. described the formation of methacrylate alendronate monomer (MA-AL. FIG. 2) and the polymerization of the MA-AL monomer by gamma radiation for wound healing. The MA-AL monomer was formed by interaction of methacryloyl chloride with alendronate (US patent application 2010/0172860). Katsumi et al described the formation of PEG-conjugated alendronate by binding alendronate to Methyl-PEO8-NHS (Katsumi. H, Takashima. M, Sano. J, Nishiyama. K, kitamura. N, Sakane. T, Hibi. T, Yamamoto. A, Development of Polyethylene Glycol-Conjugated Alendronate, a Novel Nitrogen-Containing Bisphosphonates Derivative: Evaluation of Absorption, Safety, and Effect After Intrapulmonary Administration in Rats. J. of Pharmaceutical Sciences 100 (9), 3783 (2011)). Benyettou et al. coated iron oxide nanoparticles with alendronate by physical adsorption of the alendronate to the surface of non-coated iron oxide particles (Benyettou. F, Chebbi. I, Motte. L, Sekskek. O, Magnetoliposome for alendronate delivery, J. of Mater. Chem., 21, 4813, 2011).